Energy absorbing device

ABSTRACT

An energy absorbing device for safely arresting moving bodies. Applications of the device include fall protection systems that protect workers on elevated structures from accidental falls. The energy absorbing device includes a coiled strip capable of dissipating energy when a tensile force applied across the device generates an internal bending strain that exceeds the elastic limit of the material. The tensile load developed on the safety track during an accidental fall unwinds the strip, thus progressively taking-up the kinetic energy of the moving body.

FIELD OF THE INVENTION

The invention relates to the field of safety equipment, moreparticularly to a novel energy absorbing device for arresting the motionof moving bodies. The energy absorbing device finds applications inpersonal fall protecting systems that protect workers against accidentalfalls. The invention also extends to a fall protection systemincorporating the novel energy absorbing device.

BACKGROUND OF THE INVENTION

To prevent fatal injuries as a result of accidental falls, labourlegislation codes require workers that perform a task on an elevatedstructure to wear a safety harness firmly attached to a fixture. In theevent of an accidental fall, the safety harness is intended to arrestthe falling movement at a safe distance above ground.

For an increased manoeuvrability it is common practice to tether thesafety harness to a horizontal or a vertical safety track, such as asteel cable or a synthetic rope, among others, anchored to the structureon which the work is performed. The safety harness is freelydisplaceable along the track allowing the individual to walk around thework site without impairing the level of fall protection. In the eventthat a fall takes place the track anchors must generate the reactionforce necessary to decelerate the human body to a stop.

The loading imposed on the various components of a fall protectionsystem can be extremely severe especially in the case where severalworkers collectively fall. In order to buffer the loading, it is knownto provide the fall protection system with an energy absorbing devicethat progressively dissipates the kinetic energy of the falling body.

The prior art discloses a variety of energy absorbers specificallydesigned for use in personal fall protection systems. The followingpatents are representative of state-of-the-art in this field.

    ______________________________________                                        PATENT NUMBER AND COUNTRY                                                                           PUBLICATION DATE                                        ______________________________________                                        U.S. 4,100,996        July 18, 1978                                           U.S. 4,446,944        May 8, 1984                                             U.S. 4,538,702        September 3, 1985                                       U.S. 5,174,410        December 29, 1992                                       U.S. 5,224,427        July 6, 1993                                            CANADA 2,039,004      March 25, 1991                                          ______________________________________                                    

Most of the energy absorbers described in the prior art noted above,dissipate energy by inducing a controlled rupturing of a fibrousnetwork. One approach consists of folding several times upon itself astrip made of non-stretchable woven fibers. The various plies of thestrip are attached to one another by sacrificial links. The suddentensile loading developed in a fall progressively breaks the links thatprovide a shock absorbing action. In a somewhat different approach, apin is forced longitudinally through a strip of woven fibers to provideenergy dissipation by producing a long and continuous rupture line inthe fibrous material. The prior art also contemplates a non-destructivedesign that absorbs energy through frictional force developed when alength of synthetic webbing is pulled through a buckle.

The prior art energy absorbers noted above suffer from a variety ofdrawbacks. The designs utilizing sacrificial links provide merely anintermittent energy absorption effect. The force/deploymentcharacteristics of this style of energy absorbers give rise to forcespikes each time a link ruptures, followed by a relatively unimpededdeployment until the next link becomes loaded. The shock absorbers thatuse a pin destructively tearing synthetic webbing are efficient energydissipators, however, their behaviour is largely dependent uponenvironmental conditions. For instance, the force necessary to inducedeployment significantly varies between a dry webbing and a wet frozenwebbing. Synthetic materials also have a limited lifespan when subjectedto ultraviolet radiation such as sunlight. The same observations canalso be made with regard to the energy absorbers based on frictionalforces. Here, the behaviour of the device also depends upon thefrictional/thickness properties of the webbing. A minor change in thefriction properties could entail a significant variation in thedeployment characteristics.

OBJECTIVES AND STATEMENT OF THE INVENTION

An object of the present invention is an energy absorbing device fordecelerating a moving body that alleviates the drawbacks of the priorart.

Another object of the invention is a fall protection systemincorporating the aforementioned energy absorbing device.

As embodied and broadly described herein, the invention provides adevice for decelerating a moving body, comprising:

a first load application site for connection to a support;

a second load application site subjected to tensile loading uponinteraction with the moving body; and

an energy dissipating member of ductile material extending from saidfirst load application site to said second load application site in avarying direction and being capable of absorbing energy upon applicationof a bending stress, said energy dissipating member having across-section of elongate shape circumscribing a compression zone and atension zone separated from one another by a neutral line that extendslongitudinally on said cross-section, application of a bending stress tosaid energy dissipating member compresses material in said compressionzone, stretches material in said tension zone while material at saidneutral line remains substantially unstressed, said zones havingrespective centroids separated by a distance exceeding D/2.35 where D isthe maximal dimension of said cross-section measured orthogonally tosaid neutral line, whereby the tensile loading developed on said secondload application site by said moving body induces a bending strain insaid energy dissipating member, the bending strain causing a plasticdeformation of said energy dissipating member that absorbs kineticenergy of the moving body.

The cross-sectional configuration of the energy dissipating memberinfluences its ability to efficiently take-up the kinetic energy of themoving body. When the energy dissipating member bends the amount ofmaterial deformation in the tension zone increases with the distancefrom the neutral line. In other words a particle located far from theneutral line is stretched significantly more than a particle close tothe neutral line. The energy absorbed in bending the energy dissipatingmember is the absolute summation of the mathematical product of theelongation on each particle of material in the cross-section times thestress of plastic yielding. This is mathematically equal to the yieldstress times the volume of material undergoing the elongation times thedistance of centroid of that volume of material from the neutral line,times the average elongation strain of the particles in the elongationzone plus the yield stress times the volume of material undergoingplastic shortening times the distance of the centroid of that volumefrom the neutral axis times the average shortening strain of theparticles in the shortening zone.

The efficiency of the energy dissipating member to take-up energy isthus equal to the total energy absorbed (when the outer particles reacha predetermined limiting elongation) divided by the total volume ofmaterial. Thus the energy efficiency is mathematically proportional tothe distance between the centroids of the elongation/compression zonefrom the neutral axis.

Energy dissipating members having cross-sectional shapes where thedistance between the centroids of the tension zone and the compressionzone exceeds D/2.35 are considered comparatively efficient and fallunder the present inventive concept. Most preferably, the distancebetween the centroids substantially exceeds D/2. This cross-sectionalconfiguration corresponds to a flat plate. By contrast, a perfectlycircular cross-section, which is considered outside the scope of thisaspect of the invention has an inter-centroid distance of D/2.356.

From a second aspect, the invention provides a device for decelerating amoving body, comprising:

a first load application site for connection to a support;

a second load application site subjected to tensile loading uponinteraction with the moving body; and

a hollow energy dissipating member (for the purpose of thisspecification the term "hollow" means an internal three-dimensionalregion substantially free of ductile material. This may be an emptycavity or a region containing material that exhibits non-ductilebehaviour. The three-dimensional region can be continuous ordiscontinuous and can run longitudinally of the energy dissipatingmember or in another direction) of ductile material extending from saidfirst load application site to said second load application site in avarying direction and being capable of absorbing energy upon applicationof a bending strain, whereby the tensile loading developed on saidsecond load application site by said moving body induces a bendingstrain in said energy dissipating member, the bending strain causing apermanent deformation of said energy dissipating member that absorbskinetic energy of the moving body.

A hollow energy dissipating member presents the advantage ofconcentrating a higher proportion of ductile material away from theneutral line in order to increase the amount of energy absorbed per unitof ductile material cross-section. As noted earlier, the ductilematerial in the vicinity of the neutral line absorbs a marginal fractionof the overall energy intake because it is stretched very little duringthe plastic deformation. By shifting this material toward the outerperiphery, more energy (for a given degree of plastic deformation) isabsorbed because an increased amount of material is being elongatedduring the bending process.

In one embodiment of the present invention the energy dissipating stripis made solely of ductile material such as steel or aluminium thatabsorbs energy when the bending strain creates a plastic deformation. Ina variant, a compound energy dissipating strip is provided using acombination of ductile/non-ductile materials joined as co-extensivestrips. The bending strain plastically deforms the ductile strip andalso induces a fracture front in the non-ductile strip that progresseslongitudinally as the energy dissipating member bends. The energyabsorption capacity of the unit is depleted when the ductile material isdeformed to the established limit and the non-ductile material ispulverized. The non-ductile material may be concrete or a polymericmaterial, among many others.

In a third aspect, the invention provides a device for decelerating amoving body, comprising:

a first load application site for connection to a support;

a second load application site subjected to tensile loading uponinteraction with the moving body; and

an elongated energy dissipating member of ductile material capable ofabsorbing energy upon plastic deformation, said energy dissipatingmember including first and second interconnected segments extendingtoward said first and second load application sites, respectively, toestablish a continuous load transmission path between said sites, saidsegments being in a convolved condition (i.e. one segment wound upon theother), whereby the tensile loading developed on said second loadapplication site by said moving body causes said segments to spread outand absorbs kinetic energy of the moving body.

In a fourth aspect, the invention provides an energy absorbing devicedeformable upon the application of kinetic energy in tension to spacedends of the device, the device comprising:

an elongate member with a longitudinal axis extending centrally of itscross-section along its length;

the longitudinal axis throughout its length lying in a common flatplane;

the member is in any cross-sectional plane normal to its longitudinalaxis along its length, symmetrical about an axis of symmetry lying insaid any cross-sectional plane;

the member having a first end and a second end spaced from the first endwith curved portions therebetween;

the curved portions comprising a plurality of curved segments with radiiof the curved segments varying along the length of the member such thatdifferent segments plastically deform under different magnitudes oftension.

In a most preferred embodiment, the first and second segments have acommon origin and are rolled together into a dual coil having a planarconfiguration, i.e. the central axes of the segments are co-planar. Thefree extremities of the segments that materialize the load applicationsites extend in opposite directions from the dual coil. Most preferably,the load application sites are on an imaginary line within the plane ofthe dual coil, the imaginary line passing through the centre of the dualcoil. This feature enables the energy dissipating member to deploystraight when loaded and absorb energy in a predictable fashion.

The energy absorbing device embodied herein is particularly advantageousfor use in the field of industrial protective equipment, For instance,the energy absorbing device can be incorporated in the horizontal or thevertical track of a fall protection system to which are tethered theindividual safety harnesses.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are generalized graphical illustrations of personal fallprotection systems using an energy absorber in accordance with thepresent invention;

FIG. 3 is a schematic illustration of a system for arresting runawayvehicles using the energy absorber in accordance with the presentinvention;

FIG. 4 is a perspective view of an energy absorber in accordance withthe present invention;

FIG. 5 is a side elevational view of the energy absorber shown in FIG.4;

FIG. 6 is a bottom plan view of the energy absorber shown in FIGS. 4 and5;

FIG. 7 is a top plan view of an energy dissipating strip suitable formanufacturing the energy absorber of FIGS. 4, 5 and 6, the strip havingdiscrete cross-sectional area variation;

FIG. 8 is a top plan view of the energy dissipating strip according to afirst variant, the strip having a continuous cross-sectional areavariation;

FIG. 9 is a top plan view of the energy dissipating strip according to asecond variant, the strip having a continuous cross-sectional areavariation;

FIG. 10 is a cross-sectional view taken along lines 10--10 in FIG. 4;

FIG. 11 is a cross-sectional view of an energy dissipating member havinga circular configuration;

FIG. 12 is a cross-sectional view of the energy dissipating strip inaccordance with a further variant;

FIG. 13 is a cross-sectional view of the energy dissipating strip inaccordance with another variant;

FIG. 14 is a side elevational view of the energy dissipating strip inaccordance with yet another variant;

FIG. 15 is a bottom elevational view of the energy dissipating strip ofFIG. 14;

FIG. 16 is a cross-sectional view taken along lines 15--15 in FIG. 15.

FIGS. 17, 18, 19, 20 and 21 show sequential steps in a method of makinga device of the type illustrated in FIG. 5 with FIGS. 17 and 18 showingthe clamping of a strip between clamp halves and FIGS. 19 to 21 showingthe rotation of the strip clamped between the clamp halves relative totwo fixed rods forming part of the forming apparatus;

FIG. 22 shows a side pictorial view of one end of a fall protectionsystem similar to that shown in FIG. 1 and including both an energyabsorption device and a secondary safety cable;

FIG. 23 shows a top pictorial view of the fall protection system of FIG.19;

FIGS. 24, 25, 36, 27, 38, 29, 30 and 31 schematically show sequentialsteps in a second method and apparatus for making an energy absorbingdevice of the type illustrated in FIG. 5;

FIG. 32 is a cross-sectional view of the energy dissipating strip inaccordance with another convoluted coiled variant; and

FIGS. 33, 34, 35 and 36 are pictorial side views of four furthervariants of energy dissipating strips in accordance with the presentinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a personal fall protection system incorporating an energyabsorbing device in accordance with the invention. The fall protectionsystem 2 includes a horizontally extending safety track 4, in the formof a metal or synthetic cable, along which are displaceable the safetyharness 6 of an individual worker. The safety track is anchored securelyto the elevated structure on which the work is being performed withanchoring brackets a of sufficient strength to withstand the forcesdeveloped in the event of a fall. In order to greatly reduce theseforces and provide a gradual deceleration of the falling human body, anenergy absorbing device designated comprehensively by the referencenumeral 10 is connected between the extremity of the safety track 4 andone or both of the anchors 8.

FIG. 2 illustrates a variant of the fall protection system 2, where thetrack has a vertical configuration. The energy absorbing device 10 isconnected between the track and a suspension point 11.

Referring now to FIG. 4, the energy absorbing device comprises a strip12 of ductile material. As seen in FIG. 4 on one side view, the elongatemember 12 has a central portion 109 of generally "S" shape. ThisS-shaped central portion may be seen to comprise the portion 109 of theelongate member 12 which extends between approximately point 101 andpoint 102. As seen in FIG. 4, one first end of the central "S" shape (asshown in FIG. 4 the lower end) merges into a respective first clockwisecurving portion 112. Similarly, the other second end of the central "S"shape (in the case of FIG. 4 the upper end) merges into a respectivesecond clockwise curving portion 113. The first clockwise curvingportion 112 may be seen to extend curving clockwise from point 102 topoint 104. The second clockwise curving portion 113 may be seen toextend curving clockwise from point 101 to point 103. In this manner, afirst end of the "S" shape merges into its respective clockwise curvingportion 112 with the clockwise curving portion 112 extending curvingclockwise to overlie initially the other, second end of the "S" shapeand subsequently the other, second clockwise curving portion 113.Similarly, the second end of the "S" shape merges into its respectivesecond clockwise curving portion 113 which extends curving clockwise tooverlie initially the first end of the "S" shape and subsequently theother first clockwise curving portion 112.

The first clockwise curving portion 112 terminates as a respective firstend portion 114 extending from point 104 to point 108. The secondclockwise curving portion 113 terminates as a respective second endportion 115 extending from point 103 to point 107. Aperture 34 in firstend portion 114 provide an attachment member at the first end of theelongate member 12 and aperture 14 in second end portion 115 provides anattachment member at the other second end of the elongate member 12.

FIG. 4 shows that the central portion 109 of "S" shape is formed of twohalves 110 and 111, each of which are of "C" shape. First half 110extends from center point 26 to point 101. The second half extends inthe opposite direction from center point 26 to point 102. The two halvesare thus joined at point 26 through which a center axis 100 passes. Itmay be seen that in side view as seen in FIG. 4, each half 110 and 111is located in an identical orientation to the other half but rotated180° about the center axis 100. It may be also seen that each of theclockwise curved portions 112 and 113 is of identical shape to the otherof the clockwise curved portions and in a side view as seen in FIG. 4,each clockwise curved portion 112 and 113 is located in identical to theother clockwise curved portion but rotated at 180° about the center axis100.

As seen in FIG. 4, first end portion 114 has a first terminal section116 between points 106 and 108 which is substantially planar as seen inside view. Similarly, second end portion 115 has a second terminalsection 117 between points 105 and 107 which in side view is planar. Aswell, FIG. 4 shows that a planar line can extend centrally through eachof terminal portions 116 and 117 and through center axis 100.

It is seen in FIG. 4 that each end portions 114 and 115 are locateddiametrically opposite to the other of the end portions

Reference is now made to FIG. 4 in conjunction with notably FIG. 5 whichdescribes strip 12 in accordance with a different characterization ofsegments of the strip 12.

Referring to FIGS. 4, 5 and 6, the energy absorbing device 10 comprisesa strip 12 of ductile material having at one end an aperture 14constituting a first attachment point of the energy absorbing device.The strip 12 includes a first curved segment 16 of stepwise decreasingradius originating near the aperture 14. Segment 16 extends fromlocation 21 to location 26. The segment 16 has an initial bend 18 ofinverted S-shape which extends from location 21 to location 22. The bend18 merges with a significantly longer intermediate bend 20 which extendsfrom location 22 to location 23. The curved segment 16 ends with aterminal bend 24 that originates at location 23 and ends at 26 locationwhich coincides with the geometrical centre of the strip 12.

The two curved sections forming the bend 18 of inverted S-shape have thesame radius. The bend 20 has a constant radius that is smaller than theradius of the curved sections of the bend 18. In a similar fashion, thebend 24 has a constant radius that is smaller than the radius of thepreceding bend 20,

The remaining portion of the strip 12 is constituted by a second curvedsegment 28 originating at location 26 and terminating at an opening 30constituting the other attachment point of the energy absorbing device10. The second curved segment 28 is the same geometrical shape as firstcurved segment 16 but rotated by 180° about a central axis (shown as 100in FIG. 4) extending through point 26 normal to the cross-section seenin FIG. 5. The bends forming the second curved segment 28 are identicalto those of segment 16 and are identified by the same reference numeralsfollowed by the suffix "a"

The segments 16 and 28 are wound in a dual concentric coil with theapertures 14 and 30 constituting the load application sites andattachment points of the energy absorber extending on either side of thecoil. It will be noted that attachment points 14 and 30 are centred onan imaginary horizontal line 31 intercepting the centre point location26 of the coil. The imaginary line 31 lies in a vertical plane (shown as99 in FIG. 10) containing the centre line 33 of the strip 12 (shown inFIG. 10). The special relation between attachment points 14, 30establishes a line of deployment of the strip 12 that is co-linear withthe tensile vectors acting on the energy absorbing device 10 in theevent of deployment. This feature allows the energy absorber to spreadout straight from both ends in a stable and highly predictable manner soas to achieve a controlled energy dissipation, as discussed in detailbelow.

The tensile loading developed in the case of a fall pulls apart theattachment points 14 and 30. The resulting bending strain induced inboth curved segments 16 and 28 unwinds the dual coil by progressivelystraightening the bends of the curved segments. If the strip 12 has aconstant thickness and width, the rate of energy dissipation is stageddue to the increasingly tighter radius of the successive bends. Statedotherwise, the amount of energy required to unwind the strip 12 by apredetermined amount is lower when the bends 20 and 20a are straightenedthan when the smaller radii curves 24 and 24a straighten. This featureprovides the shock absorber with the ability to take-up at anincreasingly higher rate the kinetic energy of the moving body as theextent of the deployment stroke increases.

Reference is now made to FIG. 10 which shows a cross-sectional view ofthe device of FIG. 4 along section line 10-10'. FIG. 10 shows as 33 thelocation of a centre line and longitudinal axis of the strip whichextends centrally of the cross-section of the strip along its length. Inthe context of the strip 12 which is a straight, flat member of uniformrectangular cross-section prior to forming into the coil shown in FIG.5, this longitudinal axis 33 would be a straight line extendinglongitudinally through the strip. In the context of the coiled device 10shown in FIG. 5, the axis 33 is shown as a dotted line centrally of thestrip 12. The strip 12 is formed such that the longitudinal axis 33 ofthe device shown in FIG. 5 lies in a common flat plane indicated as 99in FIGS. 10 and 6.

FIG. 5 shows location 26 as the centre of the coiled device 10. FIG. 4illustrates a central axis 100 which extends through location 26 normalto longitudinal axis 33 and normal to common flat plane 99. As pointedout earlier, the first curved segment 16 and the second curved segment28 are substantially identical but displaced 180° relative to each otherabout central axis 100.

In the device of FIG. 5, the strip 12 has a regular rectangularcross-section as seen in FIG. 10. With such a strip 12,

it will be apparent that in any cross-sectional plane normal to thelongitudinal axis 33, the strip will be symmetrical about an axis ofsymmetry lying in that cross-sectional plane. Thus, in the context ofFIG. 10, the strip in the cross-section shown normal to axis 33, thestrip is symmetrical about an axis of symmetry lying in thecross-sectional plane and lying in common central plane 99.

For applications in which it is desirable to maintain a generallyconstant deployment force during the entire extension stroke of theenergy absorbing device 10, the embodiments shown in FIGS. 7, 8 and 9can be used. In FIG. 7, the effect of the tighter radius on themagnitude of bending strain required to produce a plastic deformation ismuted by diminishing the cross-sectional width of the strip 12. Morespecifically, the strip 12 has a discrete variation of its transversedimension, the variations occurring at the boundaries between theinitial, intermediate and terminal bends of each strip. This embodimentis most suitable where the radius of the bends varies discretely, as inthe embodiment of FIGS. 4 to 6.

FIG. 8 depicts an embodiment characterized by a continuous transversedimension variation. This feature is suitable for a strip 12 having aradius that diminishes continuously from one attachment point toward thegeometric centre of the dual coil. FIG. 9 is a variant where thecontinuous cross-sectional reduction near the centre of the strip 12 isachieved by stamping out a slot 32 that tapers toward the anchoringpoints 14,30.

FIG. 10 illustrates the bending strains induced in the strip 12 when theenergy absorber device 10 is subjected to tensile loading. Essentially,the cross-section of the strip 12 is divided in two zones: zone 34subjected to tension and zone 36 undergoing compression. The zones 34and 36 are separated by a neutral line 37 coinciding with thelongitudinal axis of the cross-section. When the strip 12 is plasticallydeformed the ductile material in tension zone 34 is stretched by anamount depending upon the distance from the neutral line 37. Forinstance, a particle on the surface of the strip elongates significantlymore than a particle located near the neutral line 37. Note thatparticles at the neutral line remain totally unstressed during bendingof the strip 12.

The present inventor has made the unexpected discovery that the energyabsorption efficiency of the energy absorber device 10 is dependent uponthe cross-sectional configuration of the strip 12. As discussedpreviously, the energy absorption efficiency is proportional to thedistance separating the centroids of the tension zone 34 and thecompression zone 36. In FIG. 11, the centroids are designated byreference numerals 39 and 41. The distance separating the centroids 39and 41 can be expressed as a proportion of the maximal transversedimension D (the dimension measured orthogonally to the neutral line37). The rectangular section such as shown in FIG. 10 has aninter-centroid distance of D/2.

FIG. 12 illustrates a circular cross-sectional shape that is lessefficient than the rectangle. It can be mathematically demonstrated thatthe inter-centroid distance for the circular configuration is D/2.356.For a given bending curvature and for a given cross-sectional area, therectangular strip having an inter-centroid distance of D/2 will absorbabout 18% more energy than the circular strip.

According to the invention, the cross-sectional configuration of thestrip 12 is such that the distance between the centroids preferablyexceeds D/235, more preferably D/2.2 or D/2.1. A configuration providingan inter-centroid distance not substantially less than D/2 is preferredsince such a configuration closely corresponds to a rectangular sectionas shown in FIG. 10. Preferably, a configuration provides aninter-centroid distance of equal to or less than about D/2.0.

To further increase the efficiency of the ductile material in absorbingenergy, designs are considered where the area in the vicinity of theneutral axis is devoid of ductile material. Such devices can absorb ahigher amount of energy for a given cross-sectional area of material anda given amount of plastic deformation than solid shapes. By shifting thebulk of the material away from the neutral axis, a greater proportion ofthe available material undergoes an appreciable stretching whichtranslates in a higher energy intake. Energy absorbers in which thedegree of bending allowed before depleting the energy absorptioncapacity is fixed by elongation, could benefit from this innovativeapproach. For instance, consider the case of the energy absorbing device10 that takes-up energy while it deploys. Once the strip 12 is totallyflattened, i.e. it has been straightened out, very little energy will befurther absorbed in bending. If desired to increase the energyabsorption capacity for the same amount of ductile material, one couldeither re-design the geometrical configuration of the device in order toincrease the proportion of material that achieves the limiting ofplastic deformation of substitute a hollow strip (while maintaining anidentical cross-sectional area) without altering any other factors. Thelatter approach is simpler and elegant.

Alternatively, device efficiency could be increased while maintainingthe same development force, overall stroke and total energy absorbingcapability by utilizing a hollow strip that achieves the same ductilebending strength as the solid plate, but has the advantage of using lessmaterial.

Cross-sectional shapes that embody the concept described above are shownin FIGS. 12 and 13. In FIG. 13, the strip displays a plurality ofelongated cylindrical cavities 38 that are parallel and spaced along thelongitudinal axis of the strip. The variant in FIG. 13 has internalrectangular cavities 39. The relative sizes of the cylindrical cavities38 in FIG. 12 or the rectangular cavities 39 in FIG. 13 could be variedalong the length of the strip as desired.

It could also be envisaged to fill the internal cavities 38,39 withsubstantially non-ductile material that would crack during bending. Theprogressive pulverisation of the non-ductile material would furtherincrease the energy absorption capacity. The non-ductile material can beany suitable substance that cracks instead of plastically deforming,such as synthetically prepared rigid resinous material, among manyothers.

For applications requiring a large energy absorption capacity theembodiment illustrated in FIGS. 15 and 16 could be employed. The energyabsorber device 10' is constituted by a strip 12' in the dual coilconfiguration made of composite material. More particularly, the strip12' includes a network of reinforcing metallic wires 40 perpendicularlycrossing each other. The wires 40 may be welded wire mesh or reinforcingbars typical for usage in reinforced concrete constructions. The wires40 are embedded in a matrix of binding agent such as ordinary concreteconsisting of graded aggregates and Portland cement. For increasedefficiency in energy absorption per volume of wire materials, the short,straight wires nay be significantly smaller than the main wiresfollowing the coiled shape since the transverse wires only serve to holdthe absorber together during deployment and cracking of the matrix anddo not directly contribute to the energy absorption.

Anchoring points 14' and 30' are provided at the extremities of thestrip 12 in the form of wires with hook ends that mechanically interlockwith the network of crossing wires 40 and are partially embedded in thematrix of bonding agent.

When the attachment points 14' and 30' are pulled apart, a bendingstrain is induced in the strip 12'. This is best shown in FIG. 16. Thebending strain creates a tension zone 42 and a compression zone 44separated by a neutral axis 46. The wires 40 may be located exactly atthe neutral axis, hence in the initial stages of the tensile loadingthey remain substantially unstressed. Note, however, that the wires 40may be moved higher in the tension zone 42 if the thickness of thesection permits. When the bending strain exceeds the elastic limit ofthe bonding agent in zone 42, the latter cracks which has the effect ofdissipating energy. Since the cracked material can no longer offer ameaningful reaction force to the tensile loading the neutral axis shiftsto a new position 46a. The resulting balance of forces causes the wires40 to stretch, thereby energy is now absorbed by virtue of plasticdeformation. One design alternative of the device of the typeillustrated in FIG. 14 is to have the reinforcing wires located withinthe device in locations other than the centre plane indicated as 46 inFIG. 15, dependant upon the relative position along the length of themember. For example, the reinforcing wires could be placed nearer tosurfaces which are radially interior surfaces. Thus, the location of thewire would change depending upon which half of the device the wire islocated in.

The ductile material used in any one of the embodiments illustrated inFIGS. 1 to 36 is preferably a metal such as structural steel, stainlesssteel, aluminum, copper, titanium or any other suitable pure metal oralloy that exhibits ductile behaviour.

Reference is made to FIGS. 17 to 21 showing sequential steps in a methodof making an energy absorbing device 10 as illustrated in FIG. 5 with apreferred apparatus.

The apparatus comprises firstly, two clamp halves 101 and 102. In afirst step as seen in FIG. 17, a straight elongate length of strip 12 ofductile material having a rectangular cross section is located betweenthe two clamp halves 101 and 102. The clamp halves are then forcedtogether in the direction indicated by arrows 106 and 108 in FIGS. 17and 18 so as to deform the strip 12 to the shape as shown in FIG. 18 byreason of the strip being engaged by the interior S-shape surfaces 110and 112, respectively of each clamp halves. The clamp halves 101 and 102are secured together by means not shown clamped about the strip 12 inthe position shown in FIG. 18.

Next, the metal strip 12 with the clamp halves 101 and 102 clamped aboutit are disposed within an apparatus such that the clamp halves arejournalled for rotation about a central axis indicated as 100 whichpasses through centre point 26 and which axis is maintained fixedrelative to fixed cylindrical forming rods 104 and 106 which aredisposed fixedly located parallel to centre axis 100 and equidistant ondiametrically opposite sides therefrom.

From the position shown in FIG. 19, the clamp halves are rotatedcounterclockwise by means not shown about central axis 100 therebyrotating from the position of FIG. 19 to the position of FIG. 20 andthen subsequently to the position of FIG. 21. In rotating from theposition of FIG. 19 to the position of FIG. 20, the strip is deformed toconform to the outside surfaces 114 and 116, respectively, of the clamphalves 101 and 102.

FIG. 20 illustrates a position in which the clamp halves have beenrotated about 140° from the position of FIG. 19. FIG. 21 indicates aposition which the clamp halves have been rotated about 230° from theposition of FIG. 19.

In the context of the apparatus such as schematically shown in FIGS. 17to 21, the shape of the clamp halves 101 and 102 determines theconfiguration of the resultant device. As shown in FIG. 18, the clamphalves when clamped together with the strip 12 therebetween presenttheir outer surfaces 114 and 116 at a constant radius from central axis100 so as to present a cylinder about which the strip is wound at aconstant radius for each approximate incremental 180° of rotation fromthe position of FIG. 19. In rotating from the position of FIG. 19 to theposition of FIG. 20, the strip assumes a constant radius cylindricalcurved section. However, shortly after rotation from the position shownin FIG. 20, the radius increases at a relatively abrupt stepdiscontinuity to a curvature increased in radius by the thickness of thestrip 12. The strip then for the next approximate 180° of rotation hasthis fixed enlarged radius of curvature where it overlaps on a curvedportion of the nested, underlying other half of the strip.

It is to be appreciated that the outer surfaces 114 and 116 of the clamphalves may be provided with other configurations than that whichprovides a cylinder when they are clamped together. For example, another preferred configuration is to provide a surface which increaseswith radius from the centre 26 an amount equal to the thickness of thestrip from the point indicated as 115 in FIG. 18 to the point indicatedas 117 in FIG. 18 so as to permit the strip 12 to wind in a helicalmanner with a constantly increasing radius initially upon the outersurfaces of the clamp halves and subsequently upon the underlying nestedcurved portions of the other halves of the strip. Such a device with asmoothly helically wound strip would have an appearance similar to thatshown in FIG. 21 and therefore is not separately illustrated.

The inner surfaces 110 and 112 of the clamp halves may also assume otherconfigurations as may be desirable.

It is to be appreciated that the clamp halves may be rotated about thecentral axis 100 to desired amounts of rotation which may be in therange of from about 30° to many full revolutions, for example, 3, 4, 5or 6 for revolutions. More preferably, the rotation will be in the rangeof 90° to about 540° and more preferably at least 180°, 270°, 360°, 450°or 540°.

The device shown in FIG. 21 has the ends 119 and 121 of the strip 12extending straight as a tangent to where the curved portion of the strip12 last curves about the other half of the strip. The device shown inFIG. 21 can have each of the straight ends 119 and 121 formed into ageneral S-shape so as to provide a device the same as that shown inFIGS. 4 and 5. It is not necessary however to have the ends or each ofthem formed into the S-shape shown in FIGS. 4 and 5.

FIGS. 22 and 23 show side and top views of a portion of a fallprotection system similar to that shown in FIG. 1 and including ananchor 8 secured to a structure 9 and a horizontally extending safetytrack in the form of a steel primary cable 4 with a shackle 130 securedto its end. The anchor 8 comprises a bracket with a flange 132 having anopening to receive the bolt holding the ends of a second shackle 136.The shackles 130 and 136 each engage attachment openings 133,134 in theends of the energy absorption device 10 and as well loops 137,139 ateach end of a secondary steel cable 138. The secondary steel cable 138while not necessary, if provided is preferably of a length ofapproximately equal to that of the energy absorption device 10 should ithave its strip 12 fully extended. The cables 4 and 138 ensure a maximumdesign load bearing capability greater than that of the energy absorbingunit. The energy absorbing unit 10 is shown as identical to that in FIG.21.

Reference is now made to FIGS. 24 to 31 which schematically show asecond method and apparatus for making the energy absorbing device asillustrated in FIG. 5. In FIGS. 24 to 31, the strip is illustrated inside view for convenience as a single solid line.

The apparatus shown in FIGS. 24 to 31 has two end clamps 120 and 121only schematically shown which are to engage and hold the ends of thestrip. The apparatus also includes two cylindrical pins 122 and 123which are disposed equidistance from the centre 26 of the strip with thepins coupled together by means not shown for rotation in unison aboutcentre 26 maintained a constant distance from the centre. Each of theclamps 120 and 121 are movable along a straight track (not shown) whichis fixed and extends from the position of the clamp 120 to the positionof clamp 121 as shown in FIG. 24 through centre 26. The movable clamps120 and 121 are moved towards centre 26 by the apparatus along the trackby means not shown as the pins 122 and 123 rotate. The movement of eachof the clamps is controlled as a function of the extent of rotation ofthe pins 122 and 123 from the starting position shown in FIG. 24. Thus,the clamps 120 and 121 permit a control winding of the strip 12 aboutthe pins 122 and 123 so as to provide various advantageous shapes andconfigurations for the resultant convoluted coil.

As shown, in progressing in sequence from FIG. 24 successively throughFIGS. 25 to 31, the pins 122 and 123 are rotated in unisoncounterclockwise about centre 26 and the end clamps 120 and 121 aremoved progressively towards the centre 26.

Reference is now made to FIG. 32 which shows another variant of anenergy absorbing device 10 in accordance with the present invention. Thedevice 10 shown in FIG. 32 could be formed with the apparatus of FIGS.24 to 31. The device in FIG. 32 is shown with a continuous air space 125separating the first curved segment 16 from the second curved segment28.

The coil shown in FIG. 32 differs from the coil shown for example inFIG. 5 insofar as the thickness of the strip 12 decreases from itscentre 26 progressively to each end. This decrease in the thicknessalong the length of the strip permits the thickness to vary with radiusso as to keep the force required to extend the device 10 substantiallyconstant throughout its elongation as may be advantageous for someapplications. The device of FIG. 32 may be formed from a strip 12 ofmetal whose thickness has been varied as by rolling the metal plate tothe desired varying thickness prior to coiling. While FIG. 32 shows thethickness progressively increasing towards centre 26, the thickness maybe varied in other manners.

The device of FIG. 32 may also be formed from ductile extrudablematerials such as aluminum by extruding the device through a dye whichhas a cross-section which is the same as the cross-sectional profileshown in FIG. 32 and then cutting the extrusion at desired widthsaccording to desired deployment forces. Extruding the energy absorbermay be particularly advantageous insofar as the energy absorber may bedesired to fit into specially designed casings or the like as may beuseful for applications such as in retaining automotive seat belts insubstitution for devices such as those taught by U.S. Pat. Nos.4,358,136 and 3,482,827.

Reference is now made to FIGS. 33 to 36 which illustrate furtherembodiments of energy absorbing device 10 in accordance with the presentinvention. Each of FIGS. 33 to 36 shows a ductile strip 12 ofrectangular cross-section similar to that shown in FIG. 6. Each device10 has curved sections between its two ends. FIG. 33 shows a strip 12configured to have a curved section of generally sinusoidalconfiguration having a first wave with upper peak 160 and lower peak 162of the same height and a second wave having peaks 164 and 166 of equalbut lower height than the peaks 162 and 164. In FIG. 33, the strip 12has curved sections with different radii provided in the first peaksthan in the second peaks.

FIG. 34 provides a second generally sinusoidal configuration and inwhich a centre point is indicated as 26. The left hand side of the stripis identical to the right hand of the strip if rotated 180° about centre26. Each of the first curved section 16 of the strip to the right handside of centre 26 and the second curved section 28 to the left hand sideof point 26 have three peaks. The outermost peaks 167,168 have thegreatest amplitude, intermediate peaks 169,170 have a middle amplitudeand the inside peaks 171,172 have the least amplitude. Each of theoutermost, intermediate and inner peaks have a different radius ofcurvature. Thus, the radius of curvature varies over the length of thestrip 12 however with each of the halves being the same albeit displaced180 degrees relative point 26.

FIG. 35 shows a simplified configuration in which the strip 12 is bentinto the shape of the symbol "omega". This shape provides at least fivepossible radius of curvature indicated as R1, R2, R3, R4 and R5. In theillustrated embodiment, R1 equals R5 and R3 equals R4 such that thedevice 10 is symmetrical although this is not necessary and it may bepreferred to provide all five radii to be different such that each willrequire different forces to plastically straighten the device.

FIG. 36 indicates another configuration of the strip which issymmetrical about point 26 and provides in effect a configuration withtwo "omega" type curves similar to the device of FIG. 35.

It is to be appreciated that the devices such as shown in FIGS. 5, 32and 36 are advantageous as more compact than the devices as shown inFIGS. 30, 34 and 35. The more compact devices have a greater ratio ofthe length of the strip when straight to the length of the resultantcurved device. Devices such as shown in FIGS. 5, 21 and 32 are preferredas they are able to deliver a constant deployment force over themajority of their deployment.

Each of the devices shown in the figures have corresponding featuresinsofar as they have a longitudinal axis similar to 33 shown in FIGS. 5and 10 which passes through the centroid of their cross-sectional arealongitudinally throughout their whole length. This longitudinal axis isrepresented by point 33 in FIG. 10. In a straight flat strip 12 as shownin FIGS. 7, 8 or 9, the longitudinal axis is a straight line. In thefinal curved forms of the devices, the longitudinal axis lies in acommon flat plane such as shown as 99 in FIGS. 6 and 10. For example, inthe devices shown in FIGS. 4, 21, and 33 to 36, the single flat planewould be a vertical plane having regard to the orientation in which eachdevice is illustrated passing centrally through the strip 12. Inpreferred devices 10 as shown in FIGS. 5, 33 and 36, end portionscarrying the attachment holes are disposed within a horizontal plane(having regard to the orientation in which each device is illustrated)passing centrally through the point 26 and including the central axis100, where appropriate.

In the context of energy absorbing devices in accordance with thepresent invention, different amounts of energy absorption and rates ofabsorption may be obtained by varying amongst other things one or moreof the type of ductile material, the shape of the cross-section of theductile material with length, the relative size of the cross-sectionalmaterial with length, the amount of non-ductile material included,and/or the degree of curvature and/or rotation.

The energy absorbing device in accordance with the present invention canbe put to practical use in many applications where a moving body needsto be decelerated or arrested. Fall protection systems of the typeutilizing a horizontal or a vertical track are good examples. Use of anenergy absorbing device in accordance with the present invention has theadvantage of reducing the design stress which must be met by anchorswhich secure a fall protection system to a structure. Unlike prior artdevices, the energy absorber according to the invention exhibits astable and predictable behaviour under a variety of environmentalconditions and it is capable of continuous energy take-up over theentire deployment stroke.

The energy absorber could also be used in highway, aircraft or elevatorssafety applications to arrest runaway vehicles. This example isillustrated in FIG. 3. An arrester cable or barrier 48 is placed acrossthe road. The arrester cable 48 is connected to suitable anchors byenergy absorbing devices 10. It will be recognized that the capacity ofthe energy absorbing devices 10 needs to be increased in order to bringit in the proper relationship with the average vehicular weight. Anincrease in arresting force can be achieved by increasing the thicknessand/or width of the strip, by utilizing a ductile material that deformsat a higher stress, and/or by decreasing the bending radius. An increasein elongation can be achieved by providing the convolved section of thestrip with more turns. An increase in deployment force and theelongation will both serve to increase the total energy absorbingcapability.

The above description of the preferred embodiments of the inventionshould not be interpreted in any limiting manner since refinements andvariations are possible without departing from the spirit of theinvention. The scope of the invention is defined in the annexed claims.

I claim:
 1. An energy absorbing device deformable upon the applicationof kinetic energy in tension to the device, the device comprising:anelongate member having two ends portions, each end portions carrying arespective attachment means for the application of tension forces, alongitudinal axis extending centrally of a cross-section along a lengthof the elongate member; the elongate member formed into a convolutedconfiguration with the longitudinal axis lying in a common flat plane,the elongate member comprising a continuous element of ductile materialextending the length of the elongate member; the device characterized inthat in one side view the elongate member having a central portion ofgenerally "S " shape with each end of the "S" shape merging into arespective clockwise curving portion which extends curving clockwise tooverlie initially the other end of the "S" shape and subsequently theother clockwise curving portion; each clockwise curving portionterminating as a respective one of the two end portions of the elongatemember.
 2. A device as claimed in claim 1, wherein the central portionof "S" shape is formed of two halves of identical shape joined at acenter axis normal the longitudinal axis and the common flat plane within said side view each half located in an identical orientation to theother half but rotated 180° about the center axis.
 3. A device asclaimed in claim 2, wherein each clockwise curved portion is ofidentical shape to the other clockwise curved portion and with in saidside view each clockwise curved portion located in an identicalorientation to the other clockwise curved portion but rotated 180° aboutthe center axis.
 4. A device as claimed in claim 1, wherein eachclockwise curving portion is of identical shape to the other clockwisecurving portion and with in said side view each clockwise curved portionlocated in an identical orientation to the other clockwise curve portionbut rotated 180° about a center axis normal the longitudinal axis andthe common flat plane.
 5. A device as claimed in claim 4, wherein eachend portion has a terminal section carrying the attachment means, thelongitudinal axis through both terminal sections lying in a planar linewhich includes the center axis.
 6. A device as claimed in claim 1,wherein each end portion is diametrically opposite the other endportion.
 7. A device as claimed in claim 3, wherein each of theclockwise curving portions spiral outwardly.
 8. A device as claimed inclaim 1, wherein each clockwise curving portion overlies the other endof the "S" shape and the other clockwise curving portion over an arc ofat least 90°.
 9. (A device as claimed in claim 1, wherein each clockwisecurving portion overlies the other end of the "S" shape and the otherclockwise curving portion over an arc of at least 180°.
 10. A device asclaimed in claim 1, wherein each clockwise curving portion overlies theother end of the "S" shape and the other clockwise curving portion overan arc of at least 270°.
 11. A device as claimed in claim 1, whereinboth clockwise curving portions curve outwardly about a central axisnormal the longitudinal axis and the common flat plane.
 12. A device asclaimed in claim 11, wherein each clockwise curving portion curves aboutthe central axis an extent of at least 180°.
 13. A device as claimed inclaim 11, wherein each clockwise curving portion curves about thecentral axis an extent of at least 270°.
 14. A device as claimed inclaim 11, wherein each clockwise curving portion curves about thecentral axis an extent of at least 360°.
 15. A device as claimed inclaim 1, wherein in any plane normal to the longitudinal axis and thecommon flat plane the elongate member is symmetrical.
 16. A device asdefined in claim 1, wherein said ductile material is metal.
 17. A deviceas defied in claim 16, wherein said metal is selected from the groupconsisting of structural steel, stainless steel, aluminum, copper,titanium and alloys thereof.
 18. A device as defined in claim 12,wherein said elongate member is in the form of an elongated strip havinga width in the common flat plane significantly in excess of a thicknessthereof normal the width.
 19. A device as claimed in claim 18, whereinthe elongate member is symmetrical in any plane normal to thelongitudinal axis and the common flat plane.
 20. A device as defined inclaim 1, wherein said elongate member has in any plane normal to thelongitudinal axis and the common flat plane a cross-section of anelongate shape with a width measured in the common flat plane greaterthan a thickness measured normal the common flat plane, the elongateshape circumscribing a compression zone and a tension zone separatedfrom one another by a neutral line that extends longitudinally on saidcross-section, application of a bending strain to said elongate membercompresses material in said compression zone and stretches material insaid tension zone while material at said neutral zone remainssubstantially unstressed.
 21. A device as claimed in claim 20, whereinsaid elongate member includes substantially non-ductile elements.
 22. Adevice as defined in claim 21, wherein said non-ductile element extendsalong the longitudinal axis in said neutral line.
 23. A device asdefined in claim 1, wherein said ductile element is configured as anetwork of crossing elongated rods at least partially embedded innon-ductile material.
 24. A device as defined in claim 1, wherein saidductile element defines an internal cavity therein.
 25. A device asdefined in claim 24, wherein said internal cavity contains a non-ductilematerial that fractures upon the application of bending strain in orderto absorb energy.
 26. A device as claimed in claim 1, wherein saidelongate member consists entirely of a ductile material.